1. Field of the Invention
The present invention relates to a longitudinally pumped color center laser with transverse auxiliary illumination and, more particularly, relates to a color center laser with lenses to direct a transverse auxiliary beam to a focal line essentially equal to a length of a crystal of the color center laser.
2. Description of the Related Art
Color center lasers contain crystals having cubic lattice structures. The crystal, or color center material, is usually an alkali halide crystal containing defects consisting of halide ion vacancies, trapped electrons and often alkali-ion impurities. FIG. 1(a) illustrates the simplest such color center, an F center (from the German word "Farbe" for color). The F center has a single electron trapped by a halide-ion vacancy. Its principal absorption band, the F band, is due mostly to a single transition 18 in the visible spectrum broadened from a line to a band by the interaction of its electron with neighboring lattice ions. This visible absorption band, in an otherwise transparent material, lends coloration to the crystal. If one of its six contiguous alkali ions is replaced by an alkali-ion impurity (e.g., an Na.sup.+ ion in a KCl crystal), the F center becomes an F.sub.A center with a structure shown in FIG. 1(b). If two of the contiguous alkali ions are alkali impurities, the F center is specified as an F.sub.B center, as illustrated in FIG. 1(c). FIG. 1(d) illustrates an F.sub.2 center, being two F centers along a &lt;110&gt; direction, i.e., two electrons plus two neighboring anion vacancies. FIG. 1(e) illustrates an F.sub.2.sup.+ center being an electron trapped by two anion vacancies, i.e., a singly ionized pair of neighboring F centers. Finally, if the F.sub.x.sup.+ center is adjacent to a substitutional alkali impurity, e.g., Li.sup.+ or Na.sup.+ in a KCl crystal, it is called an (F.sub.2.sup.+).sub.A center.
An important class of color center laser materials consists of alkali halide crystals using the emission of (F.sub.2.sup.+).sub.A centers. These (F.sub.2.sup.+).sub.A centers often can be created from (F.sub.2).sub.A centers by exposure of the crystal to visible or near-ultraviolet (NUV) light. Lasing can be obtained from these (F.sub.2.sup.+).sub.A centers which are dynamically stabilized through trapping of F.sub.2 -center electrons by F.sub.A and F.sub.B centers. A technique for exposing these crystals to create (F.sub.2.sup.+).sub.A -color center lasers is disclosed in U.S. Pat. Nos. 4,301,426 and 4,519,082 by Irwin Schneider, which are specifically incorporated herein by reference. This class of color-center materials provides laser action which is efficient, capable of pulsed or continuous operation and is broadly tunable. For example, a color center laser using this class of materials is tunable from 1.6 to 1.9 microns (.mu.m) in Na doped KCl and 2.0 to 2.5 microns in Li doped KCl. By including all known (F.sub.2.sup.+ ).sub.A color center lasers, the range of tunability extends from 1.6 to 4.0 microns. However, one major drawback of previous (F.sub.2.sup.+).sub.A lasers was that their output powers have never exceeded a few hundred milliwatts.